The invention relates to the hydrocracking of vacuum gas oil or various other typical hydrocracking feedstock oils or mixtures thereof.
In hydrocracking technology, reactor operating conditions are dictated either by product quality requirements or by catalyst life. It is impossible to optimize processing conditions in a single reactor because operating conditions in the reactor are set by the most difficult components of the feed. For example, the conditions in the reactor could be set by the amount of nitrogen in the feed. Typically, in the first reactor treating raw feed, conditions are severe (high-temperature) and not conducive to aromatic saturation. Moreover, once products are formed from hydrocracking reactions, they compete with the heaviest fractions of the feed (nominally 700xc2x0 F.+ material) to gain access to the active catalyst sites. Occlusion of the products (700xc2x0 F.xe2x88x92 material) from the active sites by the heavy products is very likely.
Consequently, for a given conversion level, single reactor systems operating at the same pressure levels as multi-reactor systems produce inferior quality products. In order to compensate for this shortfall in product quality, units are run at higher pressures and with lower space velocities. In most cases, there is considerable giveaway in product quality for at least one major product especially at start-of-run conditions, as operators select an operating pressure level to guarantee the quality of all products and extend the catalyst run length. For example, the hydrocracked Jet/Kerosene Smoke Point is often 30 mm at start-of-run when the specification requires 20 mm. Similarly, the hydrocracked Diesel Cetane Index is often around 60 when the required value is 50. This product quality giveaway translates to a waste of hydrogen. In most refineries, hydrogen is an expensive commodity.
The present invention relates to a hydrocracking and hydrotreating process which minimizes hydrogen consumption and reduces the overall reactor and catalyst volumes for a given level of performance for the production of diesel oil and lighter materials including kerosene and naphtha. The process provides a first hydrogenation reaction which is limited to the hydrogenation level needed for hydrotreating the feed for the reduction of sulfur and nitrogen and for aromatic saturation and for the hydrocracking to form the diesel and lighter materials. The uncracked heavy fraction that does not require hydrogenation beyond the sulfur and nitrogen removal and aromatic saturation is separated and bypassed around a second, post-treatment hydrogenation in which only the diesel and lighter materials are further hydrogenated thereby reducing the hydrogen consumption. The objects of the invention are accomplished through the use of a main catalytic reactor operating at conditions which produce an effluent of hydrocracked oil and uncracked heavy oil followed by an intermediate vapor/liquid separator and a post-treatment reactor involving reactive distillation for final hydrocracking and hydrotreating. The primary reaction achieves a partial level of conversion without meeting final product quality with the post-treatment reaction operating to hydrogenate only the separated distillates to meet final product specifications. The invention also allows for advantageous feed locations for certain specific feed materials.